use static constexpr and braced initialization in policy.h

Summary:
This is a backport of core#25388

Most of the PR is related to chained transaction limits, so not applicable to us.

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15038